Thick film resistors are employed in hybrid microcircuits to provide a wide range of resistor values, generally between about 0.1.OMEGA. and about 10M.OMEGA.. Such resistors are printed on a ceramic substrate using thick-film pastes, or inks, which are conventionally composed of an organic vehicle, a glass frit composition, an electrically conductive material, and various additives used to favorably effect the final electrical properties of the resistor. The organic vehicle determines the flow characteristics of the ink, while the glass frit composition primarily serves to adhere the electrically conductive material together, as well as bond the resistor to the substrate.
After printing, thick-film inks are typically dried by infrared radiation at temperatures of about 150.degree. C. Thereafter, the printed pattern is sintered, or fired, to convert the ink into a suitable film which adheres to the ceramic substrate. Sintering typically occurs by transporting the printed pattern on a conveyer through a convection furnace. A time-temperature profile for a conventional sintering process is illustrated in FIG. 1a. The process is controlled to produce a heating rate of about 100.degree. C. per minute, which is sufficiently slow to promote stability of the resistor and to allow the organic vehicle of the ink to burnoff. During sintering, which typically occurs at peak temperatures of about 825.degree. C. to about 1000.degree. C. for a duration of about 10 minutes, both physical and chemical changes occur to form the conduction network or microstructure of the resistor. Such changes involve a solidus to liquidus phase change for the glass frit composition, crystal growth of the conductive material, and changes in the oxidation state of the conductive material. The time and temperature relationships where these events occur determine the final microstructure of the resistor film, which in turn determines the resistivity, stability and temperature characteristics of the resistor. Various additives are used to shift the time and temperature relationships to achieve specific desired resistivity (.OMEGA./.quadrature.), stability and temperature characteristics.
Typically, inks are commercially available in composition families referred to as end-members, which are formulated to produce resistors having sheet resistances in decade values from about 1 ohm per square (.OMEGA./.quadrature.) to about 10 megohms per square (M.OMEGA./.quadrature.), (per 25 micrometers of dried thickness). Compositions having values which are one decade apart are referred to as adjacent end-members, which are blended to produce intermediate values of resistance. In addition, the resulting thick film resistors can be trimmed to increase their resistance values. Final resistance values of about .+-.1% can be achieved by trimming using abrasive or laser techniques.
However, the electrical resistance of a thick film resistor will vary with temperature, and may be permanently altered when subjected to a hostile environment. Such adverse effects are illustrated in FIG. 1b, which is representative of thick film resistors that have been laser trimmed to increase their resistances by about 30% after sintering. As can be seen in FIG. 1b, the change in resistance of a thick film resistor may average about 0.4% or more after being subjected to the test conditions indicated.
A thick film resistor's sensitivity to temperature is indicated by its temperature coefficient of resistance (TCR), as measured in parts per million per degree C (ppm/.degree.C.). Thick film resistors can typically be calibrated to have a TCR in the range of about .+-.50 to about .+-.100 ppm/.degree.C. Calibration to a tighter limit is generally prevented by a significant difference in the values for TCR obtained at -55.degree. C. and 125.degree. C., which are standard temperature extremes used by the industry to evaluate the electrical characteristics of thick film resistors, as well as blending anomalies which occur as a result of interactions between the additives included in the ink to selectively alter the electrical characteristics of the resistor. Such blending anomalies are particularly likely to occur when blending two adjacent end-members to obtain an intermediate resistance value. Such additives, which include noble metals and their compounds, refractory fillers, various glass frit materials, and modifiers, are conventionally added to end-members because they are capable of optimizing the performance of each end-member individually. The prior art which generally illustrates this approach includes U.S. Pat. No. 3,329,526 to Daily et al., U.S. Pat. No. 3,304,199 to Faber et al U.S. Pat. No 3,324,049 to Holmes et al., and U.S. Pat. No. 3,916,037 to Brady et al. However, such formulation techniques do not fully consider possible interactions between additives of two adjacent end-members. FIG. 1c provides an example of the variation in TCR which may occur when blending two adjacent end-members to form a resistor having a sheet resistance which is intermediate that of the two end-members. The TCR values are plotted for the -55.degree. C. and 125.degree. C. test temperatures conventionally used. As can be seen in FIG. 1c, a thick film resistor's TCR value vary significantly between the lower and upper test temperatures, such that conventional ink compositions cannot be readily formulated to exhibit low TCR values over a broad operating temperature range. TCR values also vary considerably over the blending range for the two end-members. Moreover, this variation is not proportional to the change in content of one end-member relative to the other, as one might be inclined to expect. Such a relationship between TCR and composition illustrates the adverse influence that interactions between additives can have on TCR values.
FIG. 1d illustrates another characteristic of prior art thick film resistors as a result of current processing techniques and formulations. An object of current thick film resistor compositions is the achievement of compositions whose sheet resistances (and therefore TCR, which is a function of sheet resistance) do not shift significantly with respect to sintering temperatures. Such a capability is necessary as a result of the inability to accurately control temperature variations within a convection furnace of the type conventionally used to sinter thick film resistors. Typically, production sintering specifications for such furnaces allow for about a .+-.10.degree. C. variation around the target peak temperature for the sintering process. Consequently, prior art end-member compositions are targeted for resistance shifts within approximately .+-.5% and minimal TCR shifts over the 20.degree. C. range, as illustrated in FIG. 1d.
However, to achieve minimal resistance and TCR shifts, tradeoffs have been made. For example, as noted previously, conventional ink compositions are formulated to include additives such that the resulting resistors have low sensitivity to the normal sintering temperature variations within conventional convection furnaces. When using a convection furnace to sinter thick film resistors, low sensitivity promotes a tight distribution around the target resistance for purposes of meeting the required TCR and TCR tracking specifications, meaning the allowable difference in TCR between two or more resistors, as well as maintaining resistance values within the allowable tolerance for allowing laser trim adjustments to achieve the desired final resistance value for the resistor. Generally, the less trimming required, the more stable the resistor to environmental conditions and also, the faster the throughput for the production process. However, due in large part to the additives used in ink compositions, the TCR values for some adjacent end-member compositions diverge significantly as the result of a change or variation in sintering temperature, producing the significant and generally unpredictable variations in sheet resistance depicted in FIG. 1d.
As a result, the electrical properties of such resistors cannot be selectively modified by altering the sintering temperature employed, because an adjustment in sintering temperature would not have a highly predictable effect on resistance, and a sintering temperature sufficient to alter the electrical properties of a resistor would typically be beyond the sintering range for a conventional ink composition. Consequently, when intervening processing conditions, such as changes in composition or local variances in furnace temperature, occur which undesirably alter the electrical properties of the resistors, current formulations for ink compositions do not readily allow for in-process modifications to bring subsequently produced resistors back within an acceptable range for the target resistance. Consequently, the use of conventional ink compositions necessitate that compositional changes be made, or that the source of the intervening factors be determined and eliminated. Consequently, significant yield losses result due to the inability to quickly bring the resistor values back into tolerance.
In addition, under the typical circumstances in which one or more different ink blends are required for a number of resistors in a given circuit, resistance values for the resistors can diverge in response to a given change in sintering parameters, as demonstrated in FIG. 1d, as a result of a chemical interaction between end-members of each ink composition. When resistance values diverge between resistors in a circuit, greater yield losses ultimately result. Furthermore, the likelihood of such an occurrence increases as the TCR values of the resistors increase.
From the above, it can be seen that present practices involving the formulation and processing of thick film resistors are generally inflexible in terms of producing resistors which can be accurately and repeatably processed to have low TCRs over a large operating temperature range. In addition, present practices do not generally allow for rapid in-process modifications which enable the resistances of such resistors to be continuously monitored and maintained within the target tolerance. Specifically, present ink compositions have been formulated to be insensitive to sintering temperatures, a processing step which could otherwise be utilized to advantageously influence the final resistance values for thick film resistors.
Accordingly, what is needed is a thick film resistor composition which enables the production of resistors having minimal TCR values, while also enabling in-process monitoring such that resistances of such resistors can be readily altered during the sintering operation to maintain production tolerance requirements.